Ingestive behavior of meat and egg-type chickens: equal sensitivity to naloxone

The efficacy of the opioid antagonist naloxone in attenuating ingestive behavior in stocks of chickens genetically selected for either meat (Rock-Cornish, RC) or egg production (Single-Comb White Leghorn, SCWL) was investigated. Because the stocks differ markedly in growth rate, two experiments were conducted to compare RC and SCWL cockerels at similar body weights and at the same age. Birds were injected intramuscularly with either isotonic saline or naloxone HCl at a dose of 5 mg/kg body weight. Food and water were offered ad libitum 15 min postinjection. In RC and SCWL stocks of similar body weight, naloxone significantly attenuated cumulative food and water intake through 210 min following the return of food and water. When administered to RC and SCWL stocks of the same age, naloxone significantly attenuated cumulative food intake for 300 min and cumulative water intake at 60 and 120 min. The relatively long-term depressions in cumulative food and water intake were attributable to significant decreases in incremental consumption within early time periods. There was no significant difference in the efficacy of naloxone in attenuating ingestive behavior when the stocks were compared at either similar body weights or at the same age. The results demonstrate that genetic selection for meat or egg production has not significantly altered opioid mechanisms regulating food and water intake in the domestic fowl.